Magnetic induction accelerator



June 6, 1950 R. WIDEROE 2,510,448

MAGNETIC INDUCTION ACCELERATOR Filed D90. 23, 1947 6 l c /I 3 5 K m /7 a Z nummu mk 7 j T T F l 22 i Um I; an i731. is

i atented June 6 UNHT 2,510,448 MAGNETIC INDUCTION AooriLERK'roIt Application December 23, 1947, Serial No. 793,501 In Germany October 4, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires October 4, 1964 Claims.

This invention relates in general to accelerators for charged particles and in particular to those of the high speed type. Although the present invention is applicable to any of the known types of particle accelerators where a stream of charged particles is accelerated along a closed orbital path, such as devices known as synchrotrons or betatrons, it will be described and illustrated in connection with an accelerator of the magnetic induction type. These betatrons or ray transformers are comprised generally of an evacuated annular tube into which electrons are introduced from an electron emissive cathode and a magnetic system which produces a magnetic field varying with time and having a space distribution such that the injected electrons are accelerated by the field along a circular orbit.

During the period of acceleration, centrifugal forces acting upon the electrons would, if unchecked, tend to drive the electrons out of the desired circular orbit and cause them to impinge upon the walls of the tube and become lost. To prevent this, the magnetic system is so designed that a part of the magnetic field threads the tube in such a manner as to create a compensating centripetal force upon the electrons thus confining the latter to a circular orbit of substantially constant radius during their entire accelerating period.

The part of the magnetic field from which the centripetal force is derived is commonly referred to as the control or guiding field while the field producing the electron acceleration is known as the inducing field. It has been established that the centrifugal and centripetal forces will theoretically balance out to maintain the electrons in a circular orbit of constant radius r (often referred to as the equilibrium circle) when the following relationship is established:

where is the magnetic flux included within the electron orbit, r is the radius of the orbit, and Hr is the field strength at the orbit.

The desired field relationship according to the above equation can in general be attained by making the reluctance of the magnetic path greater by an appropriate amount at the orbit than its average reluctance within the orbit.

According to another and generally accepted theory, the gradient of the magnetic field in a radial direction at the control field poles can be so designed that any tendency on the part of the electrons to deviate from the prescribed equilibrium orbit either in an axial or radial direction or a combination of both instantly gives rise to stabilizing forces which drive the errant electrons back into the orbit. Normally the desired field gradient is attained by tapering the control field poles outwardly so that the gap between them increases in a radially outward direction.

From a practical operating point of view, however, it has been learned that the stabilizing forces obtained by regulation of the field gradient at the control poles are not always sufficient and'it has been proposed to supplement them with other stabilizing forces such as by means of an annular lens way placed concentrically cation, either type of lens way and the main tube are gradually enlarged at one section to accommodate a tangentially arranged branch tube through which the electrons are introduced to the acceleration orbit. The respective diameters of the lens way and tube are greatest where the branch tube enters the main tube and thereafter decrease in the direction of electron flow until a constant diameter is reached, the latter being then maintained for the remainder of the tubes circumference. It is evident that with such an arrangement the radially directed stabilizing forces produced by the lens way are weaker for that part of the tube and lens when the latter are necessarily enlarged than they are over the remainder of the tube circumference where the tube and lens radii are much smaller and of constant value.

The electron stream completes many thousands of revolutions on the acceleration orbit between the time it is first introduced and the time when it reaches its final velocity, and thus undergoes a periodically changing stabilizing force during each revolution as it passes through that portion of the lens way where the lens diameter varies. Since the stabilizing force, viewed in the direction of electron travel, diminishes abruptly at the place where the electron stream enters the main tube body and thereafter gradually increases as the tube and lens diameters decrease, the periodic curve of the force contains not only a fundamental wave caused by the period of electron rotation but also very much higher harmonies. The electrons can also be thought of as describing oscillatory movements with respect to the axis of the equilibrium circle since the lens force directed radially towards the axis is zero at the equilibrium circle itself and increases with distance from the circle. Stabilization of the electron stream can therefore be viewed as a constant oscillation of the electron in a direction transverse to the circumference of the acceleration orbit.

If the characteristic frequency of the electrons oscillating in this manner coincides with the fundamental wave above referred to or with one of the hormonics of the varying stabilizing force, a condition of resonance will obtain which may result in such a great increase in the oscillation amplitude that a great part of the electron stream will be lost by impingment of electrons against the walls of the tube.

The object of the present invention is to provide an improved construction for the lens way in which the stabilizing forces remain uniform throughout the entire circumference of the tube. The new arrangement which also entails a redesign of the electron emissive cathode thus avoids the undesirable periodic change in stabilizing force inherent with the arrangement described in my prior application and materially improves the overall stability of the electron stream during its acceleration.

Generally speaking, the new construction consists of an annular tube of substantially uniform diameter throughout its entire circumference. Stabilizing fields alternating 'in polarity surround the tube and these are arranged concentrically with respect to the tube center i. e. the axis of each field coincides with a tangent at equilibrium orbit in the same manner as described in the previously mentioned co-pending application. The cathode for producing the electron streams at the proper point in the cycle of the current Wave that provides the cyclically varying inducing and control fields is, however, quite different and consists of one or more annular bodies placed in the tube perpendicular to a tangent to the equilibrium orbit and equidistant between two adjacent stabilizing fields. The center of the cathode coincides with the equilibrium orbit. The new construction includes in addition a tubular electrode also concentric with the orbit and placed directly in front of the cathode so as to form with the latter an electrostatic immersion lens for properly guiding the emitted electrons into the prescribed equilibrium orbit.

A preferred constructional embodiment of the invention is illustrated in the accompanying drawings wherein Fig. 1 is a vertical view in diametrical section taken on line ll of Fig. 2; Fig. 2 is a plan view of the tube and part of the magnetic field structure and Fig. 3 is-a-n enlarged sectional View in development of that portion of the tube containing the cathode to more clearly show structural details.

Referring now to the drawings and to Fig. l in particular, the electron accelerator is seen to include a central inducing core comprised of upper and lower magnetizable poles ll-l l of cylindrical form coaxial with the axis a-a of an evacuated annular glass electron tube II) which surrounds it, and the control field is produced by annular magnetizable poles 8-8 which confront one another at the electron orbit k in the tube, one of the control field poles being located above tube IE3 and the other below it. The magnetic circuit is completed by yokes ,7 interconnecting the induction field poles Ill l" and the control field poles 88, and the varying magnetic field required for accelerating the particles around orbit is is furnished by upper and lower annular coils 6'-8' which surround the control field poles and are connected to a suitable source of relatively low frequency alternating current applied to terminals 5. stabilizing fields are, in the illustrated construction, produced electromagnetically by a plurality of coils I2 surrounding the tube Ill in spaced relation. These coils are connected in series for energization from a suitable source of power such as battery l3 and it will be seen from the directional arrows in the figure that adjacent coils are wound in opposite directions so that the magnetic fields produced thereby are oppositely poled.

As more clearly shown in Fig. tube ii] at a point midway between two of the field coils I2 is enlarged to provide a bulbous cavity iiia within which is placed an annular metallic cathode body It. As previously explained this body is arranged perpendicular to a tangent to the electron orbit is and its geometrical center coincides with the orbit. The electrons themselves are emitted in the direction of the arrows from a concave inner annular surface portion I5 of the body is coated with an electron emissive material it. In lieu of the coating, an electron emissive filament in annular form may be placed at the concave annular surface l5.

Also located within the tube ill adjacent its inner wall is a metallic tubular electrode l'I. One end of the latter terminates within the bulbous tube portion Illa adjacent the annular electron emissive coating l5 and the mouth of the tube I1 is flared outwardly at It. Tube ll functions as an anode for the emitted electron stream and thus has a potential applied thereto from a source it that is positive with respect of the potential applied to the cathode coating [6. As an example, the potential at anode ll can be 0 kv. with the cathode coating it at -30 kv. The anode I! which is connected to the earth potential is also in contact with the resistive conductive coating 23rapplied to the inner surface of the vacuum tube in order to avoid charges occurring on the wall which could disturb the electron stream. This conductive coating 23 extends almost along the entire inner surface and ends shortly before the cathode It as shown in Fig. 2.

Anode H in conjunction with the cathode coating l6 constitutes an immersion lens, and with the cathode coating it, at a negative potential with respect to anode, El, the paths taken by the annular electron discharge from coating I6 will be substantially as indicated by the dashed lines 26. The electron discharge appears to originate from an imaginary annular cathode body 21 located radially inward from and in back of the real cathode l6 and thus makes it possible to introduce the electron stream into the tube H] for acceleration along the circular orbit 7c in the direction indicated by the arrow and at the same time maintain a uniform diameter of the magnetic lens way that assures uniform and alternately poled magnetic stabilizing fields around the complete circumference of the tube l0.

Because of the fact that the cathode coating I6 is located midway between two adjacent coils The alternately poled [2 of the lens way where the electron stabilizing force is zero, no difficulty is encountered in guid ing the electron stream through the immersion lens and the stream will coincide with the ac" celeration orbit is after a short run without receiving any kinetic oscillatory energy from the adjacent electromagnetic lens fields.

As the electron stream is accelerated round and round along the circular orbit is the immersion lens, formed by the cathode coating It and anode l7, will efiect a retarding force on the stream each time the latter passes through it but the initial magnitude of this disturbance can be kept low by making the cathode coating l6 narrow in the direction of the orbit 7c and placing the mouth I8 of the anode I! close to the coating such that the distance therebetween as well as the width of the coating is small in relation to the radius 1' of the tube I0. Furthermore it should be noted that the magnitude of the disturbance due to the presence of the immersion lens diminishes rapidly as the speed of the electron stream increases.

As a further measure in reducing the disturbing effect of the immersion lens upon the electron stream, the potential on the cathode coating it can normally be maintained at that of the anode l1 and brought to its negative value only for a brief period each time that an electron stream is discharged from the coating is into the circular orbit k. This can be accomplished as shown in Fig. 2 by means of a switch 22 which when thrown from the dashed line position to the solid line position indicated in the drawing raises the potential on coating It from its negative value to ground level which is also the fixed potential of anode ll. With an induction accelerator rated at mv., for example, the disturbance due to the immersion lens can be limited to about 1 microsecond or less, i. e. for at most the first hundred or less revolutions of the electron stream.

For purposes of illustration, the switch 22 has been shown schematically only. In actual practice, the switch would be controlled electronically through conventional timing circuits properly correlated with the current wave of the alternating current source that supplies the main inducing and control field components.

In addition to the advantages already discussed, the new construction permits the safe use of a very high negative potential between the annular cathode coating l6 and the anode il thereby assuring a rapid and accurate run for the electron stream from the coating into the equilibrium orbit k.

If desired, more than one cathode can be used in which case the additional cathodes would be placed at various points around the circumference of tube In in the same manner as shown in Fig. 2. Furthermore, anode ll need not be limited to a single tube but can be divided up axially into a plurality of tube sections each of which would be carried at a different positive potential with respect to that of the cathode.

In the present application, the invention has been described with respect to its application to an induction accelerator of the half wave type in which an electron stream is injected into the tube and accelerated in one direction (clockwise) around the tube once for each complete cycle of the applied alternating current wave. To adapt the invention for use on a full wave type of accelerator wherein separate electron streams would be accelerated in opposite directions of rotation about the tube on successive half cycles of the current wave it would be necessary to provide a like annular electron emissive coating or filament to the opposite face of the annular cathode carrier and a second anode like anode I! associated therewith to form the immersion lens for introducing the other electron stream.

With a full wave accelerator, half of the cathode emission would then be lost for each half wave of operation but this can be easily tolerated in view of the large cathode surface made available by the annular arrangement of the electron emissive coating or filament.

In conclusion, I wish it to be also understood that while an electromagnetic type of lens Way around the tube has been illustrated, it can be replaced with an electrostatic type without departing from the spirit and scope of the invention as defined in the appended claims.

I claim:

1. The combination with an accelerator for charged particles of the type comprising a tube within which charged particles such as electrons may follow an orbital path and means including spaced pole pieces for producing a cyclically varying magnetic field of such space distribution as to confine the electrons to a fixed orbit while continuously accelerating them along said orbit, of a uniformly sized annular lens way enclosing said orbit for stabilizing the particles during their acceleration. said lens way having its axis coincident with said orbit and comprising a plurality of circumferentially spaced electric lenses alternating in polarity, an annular emissive surface for said particles located interiorly of said tube midway between an adjacent pair of said lenses, and a tubular anode within said tube and spaced from said surface in the direction of particle emission, the respective axes of said surface and said anode being coincident with said orbit to thereby form an electrostatic immersion lens through which said particles are led into said orbit from said surface.

2. A magnetic induction accelerator as defined in claim 1 wherein the electron emissive face portion of said cathode is constituted by a surface that is concave in the direction of electron flow.

3. A magnetic induction accelerator as defined in claim 1 wherein said lenses are constituted by a plurality of coils oppositely wound lproducing magnetic fields alternating in polarity.

4. A magnetic induction accelerator as defined in claim 1 wherein said cathode is normally maintained at the same potential as said anode, and further including means for temporarily dropping the cathode potential to a negative value with respect to the anode potential during the electron emission phase of the operating cycle.

5. A magnetic induction accelerator as defined in claim 1 wherein the electron inlet end of said anode is flared.

ROLF WIDERGE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,245,670 Hollmann June 17, 1941 2,297,305 Kerst Sept. 29, 1942 

